Calendar update

A friend of mine is using a next cloud calendar with a thunderbird interface on the computer and the nexcloud app on his phone.
I sent him few months back an invite to a recurring event.
I have since erased that event. It is synced well on his phone, but the online calendar still has the original shared event and when he tries to sync Thunderbird the software freezes.

Could you please help with an advice. How can I delete this event from the next cloud online platform?